1. Field of the Invention
This invention relates to providing an output control signal for automatically switching the function of a terminal pad on an integrated circuit chip to function as either an input terminal pad or an output terminal pad in response to the detection of the presence or absence of an on-chip generated oscillator signal. More specifically, the invention relates to a detect circuit which detects the absence or presence of an oscillator signal, presumably a crystal controlled oscillator, thereby detecting the presence or absence of a crystal connected to the oscillator circuit.
2. Description of the Prior Art
In the field of digital logic, a requirement for synchronization between various circuits is generally required. Typically synchronization between these various circuits can be obtained through the use of a common clock signal. It is frequently the case in integrated micrologic circuitry, where the integrated circuit chip will have mounted upon it a crystal which is a part of a high frequency digital clock. When this circuit or chip is required to function synchronously with other integrated circuit chips, the requirement for synchronization between the chips poses a decision as to which of the various chips is to function as the master clock chip, and which is to function as a slave chip with reference to the clock signal. Where it is not possible to foresee the eventual application to which the chip is to be applied, a near redundant design of the same chip is typically required, one designed for master-mode operation and the other mask designed for slave-mode operation. Another method of overcoming this uncertainty of mode, is to have those chips which do require an externally supplied clock signal, to be connected to the terminal pad of the chip in question dedicated specifically to function as an output terminal pad, and where the chip in question is to receive an externally supplied clock signal to its circuitry, to have a separate dedicated pad for the receipt of an input clock signal. This requires additional pad space or area on the chip which in chip design is usually at a premium. Another method of providing the necessary synchronization is to visually inspect the existence of a crystal mounted on the circuit chip in question, and if the crystal is mounted to then designate the chip as a master-mode chip and therefore select a master-mode chip with a crystal plugged into it. On the other hand, if by visual inspection a crystal is not to be used, then a slave-mode type of chip may be selected which functions in a slave mode, i.e., to function in a mode which receives an input clock signal from an external source.
Logic system design would be simplified, if there were some means for the chip itself to automatically enable it to function in either a master mode or slave mode, if it were capable of self-detecting the presence or absence of the cyrstal plugged into the oscillator circuit. If this were possible, then the detection of the existence or absence of the crystal could be used to control an output driver circuit for a clock signal to either function as an output driving circuit or to float that output terminal so that an external clock source could be connected to that terminal pad, without having interference from an internally generated clock. It would increase the versatility of such a chip because then it could be automatically used as either a master-mode or slave-mode chip without visual inspection or changing the mask set design for the chip to provide both a master-mode and a slave-mode chip depending on the application.